Add unregisterFont

Provide a way to unregisterFont

## Issue or Feature
Currently it's possible to register font but not to unregister it.
I use different temporary paths for fonts that get deleted after their usage.
I tried calling registerFont multiple times hoping that would use the last provided path but for some reason it keeps references to the previous registered font (same family, style and weight)

## Steps to Reproduce
- Download font file 1
- Call registerFont
- Render
- Delete font file 1
- Download font file 2
- Call registerFont with same font settings (family, style, weight)
- Render

I get those warnings:
```
(process:1096958): Pango-WARNING **: failed to create cairo scaled font, expect ugly output. the offending font is 'Bowlby One SC Ultra-Condensed 142.5'

(process:1096958): Pango-WARNING **: font_face status is: file not found

(process:1096958): Pango-WARNING **: scaled_font status is: file not found

(process:1096958): Pango-WARNING **: shaping failure, expect ugly output. shape-engine='PangoFcShapeEngine', font='Bowlby One SC Ultra-Condensed 142.5', text='IN '
```

## Your Environment
```
% node -v
v13.7.0
% npm list canvas
@storyteq/node-common@3.2.0 /home/user/Projects/node-services/packages/node-common
├── canvas@2.6.1
```

Start by tracing the existing registerFont entry point in node-canvas and how registered fonts are retained across calls. Define the unregisterFont behavior around the reported temporary-font lifecycle, then verify that registering a replacement font no longer uses the deleted file and that rendering completes without the reported warnings.

Written by the indexing model from the issue text.